The present invention relates to an automatic gain control circuit used for the reception of multilevel duo-binary AM/PSK (amplitude modulation/phase shift keying) modulated signals in the form of bursts as defined by IEEE 802.4 Draft H, August, 1987.
According to the Draft H, three levels, i.e. levels 0, 2 and 4, are defined for a multilevel. Data are transmitted in a frame format which consists of a preamble used to regenerate clock pulses and acquire a signal level, a start delimiter indicative of the beginning of the frame, a data section, and an end delimiter indicative of the end of the frame. While the preamble is constituted of a repetitive pattern of levels 4 and 0, the start and end delimiters are constituted a the combination of levels 0 and 2.
When a receiver with a prior art automatic gain control circuit is applied to such multilevel modulated signals in the form of bursts, there arises a problem that the receiver is incapable of accurately demodulating multilevel modulated signals because the operation of the automatic gain control circuit is too slow. More specifically, when receiving a multilevel modulated signal, prior art automatic gain control circuits operate to maintain amplitude level 4 constant first, on the basis of amplitude level 4 of a preamble. Then, in response to a start delimiter which includes levels 0 and 2, the circuit tends to operate to maintain levels 0 and 2 constant based on the levels 0 and 2, respectively. However, since the operation of the circuit is slow as mentioned above, the circuit soon fails to output the levels 2 and 4 accurately. As a result, the receiver becomes unable to identify the start delimiter and therefore data which will follow the start delimiter becomes inaccurate.